Shape modification and reinforcement of columns confined with FRP composites

ABSTRACT

Strengthening reinforced concrete columns by using Fiber Reinforced Polymer (FRP) composites can be an effective method of retrofitting existing columns. FRP composites have a number of advantages over steel, including their high strength-to-weight ratio and excellent durability. The confinement effectiveness of FRP materials for rectangular sections can be improved by performing shape modification such that a rectangular column section is modified into a shape that does not have 90 degree comers such as an elliptical, oval or circular column. An expansive concrete can be advantageously used between the FRP material and the existing concrete in order to post-tension the FRP material circumferentially and improve confinement of the concrete. A finite element analytical model is also disclosed which model describes the stress-strain relationship for the FRP-confined columns after shape modification.

RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationNo. 60/610,265 filed on Sep. 15, 2004, and U.S. Provisional PatentApplication No. 60/640,545 filed on Dec. 30, 2004, each of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

In recent years, fiber reinforced polymer (FRP) composites have emergedas an alternative to traditional materials for strengthening andrehabilitation of structures. The light weight of FRP, high-strength toweight ratio, corrosion resistance, and high efficiency of constructionare among many of the advantages which encourage civil engineers to usethis material. FRP composites have been used in the retrofit of bridgecolumns due to insufficient capacity or displacement ductility. FRPjackets can provide lateral confinement to the concrete columns that cansubstantially enhance their compressive strength and ultimate axialstrain. One of the most significant problems which concerns civilengineers is the constitutive law of FRP-confined concrete. Due to theincreasing need for repair of structures, research has been carried outto investigate the behavior of FRP-confined concrete columns.

Many researchers have introduced stress-strain models to replicatecompressive behavior of FRP-confined concrete. Some studies describe thebehavior of the FRP concrete in terms of the properties of the concretecore and the confining FRP jacket. Other studies are governed by priorconstitutive models and models for steel confined concrete.

Recently, some researchers proposed a confinement model, which is basedon the concept of a variable strain ductility ratio. The researcherssuggested that the compressive behavior of FRP-confined concrete can beseparated into a strain-softening and a bilinear strain hardeningcomponent. Such a model shows agreement with experimental results forcircular columns.

Researchers have also presented a simple design-oriented stress-strainmodel for FRP-confined rectangular columns based largely on a databaseof existing test results. In this model the concept of the equivalentcircular column is introduced.

Most of the models mentioned above only refer to circular sections;moreover, test results are largely based on the standard 6 in.×12 in.concrete cylinder tests. However, for FRP-confined rectangular columns,FRP jackets provide a non-uniform confinement over the cross-section andonly a portion of the concrete section is effectively confined. For thisreason, much less is known about the behavior of FRP-confinedrectangular sections.

Unfortunately, current efforts tend to have limited value whenreinforcing rectangular columns. Further, although many reinforcingmethods result in moderate improvements in mechanical properties, thecosts of such methods tends to outweigh the benefits and increase instrength. Therefore, materials and methods for further enhancingmechanical properties of structural columns and members continue to besought.

SUMMARY OF THE INVENTION

It has been recognized that development of improved materials andmethods for strengthening structural columns which avoid many of theabove deficiencies would be a significant advancement in the industry.Accordingly, the present invention provides fiber reinforced polymer(FRP) composite structures which include a core and a fiber reinforcedpolymer material at least partially surrounding the core. The coreincludes an inner cement structure at least partially surrounded by anouter cement structure. The outer cement structure comprises or consistsessentially of expansive cement or non-shrink cement. Typically, this isthe result of applying the principles of the present invention toreinforce existing structures. Frequently, the inner cement structure ofnon-expansive cement can include steel reinforcements. However, thematerials and methods of the present invention can significantly reduceor even eliminate the need for steel reinforcement in cement structures.

In another aspect of the present invention, the inner cement structurecan have a cross-sectional shape which is different than across-sectional shape of the core. This results in a composite structurehaving a non-uniform gap thickness or non-uniform thickness of thenon-shrink or expansive portion of the core. Thus, a pre-existingstructure having, for example, a rectangular cross-section can bemodified into a structure having a circular, oval or ellipticalcross-section. Modification of the cross-sectional shape can havemultiple advantages. The elimination of corners can reduce stressconcentration and early failure of the FRP jacket. Typically, the FRPshell is cured before the grout is poured in the space between theexisting column and the FRP shell. Therefore, the effect on the FRPshell is a post-tensioning, and the effect on the existing column isradial compression. For example, the FRP materials and post-tensioningof the FRP jacket can provide improved mechanical properties asdescribed in more detail below. Additionally, elliptical, oval andcircular shapes can provide a greater degree of strength underasymmetric loads than comparable rectangular configurations.

In accordance with an embodiment of the present invention, the FRPjacket can be post-tensioned and have a hoop stress along the FRPmaterial. In one aspect, post-tensioning of the FRP jacket can bereadily accomplished by using expansive concrete. The post-tensioninginduced in the present invention can be in the form of tensile stressalong the FRP fibers, i.e. circumferential rather than axial.

In yet another aspect of the present invention, the FRP material caninclude a fiber and a polymeric matrix. Typical fibers can include, butare not limited to, glass fiber, carbon fiber, aramid fiber, andcombinations thereof. Glass and carbon fibers tend to be cost effectiveand provide good mechanical properties. Aramid fibers are light, durableand are known to have high tenacity. The selection of the fiber can bebased on factors such as cost, strength, rigidity, and long-termstability. Additionally, each type of fiber offers different performancecharacteristics and suitability for various applications. For example,aramids may come in low, high, and very high modulus configurations.Carbon fibers are also available with a large range of moduli; withupper limits nearly four times that of steel. Of the several glass fibertypes, glass-based FRP reinforcement is least expensive and generallyuses either E-glass or S-glass fibers. The fiber material for use in FRPcan be provided as sheets which can be cut to a desired size or aslengths of fiber which can be wrapped and/or laid as desired to form aparticular shape.

The polymeric resins used as the matrix for the fiber are usuallythermosetting resins. Most available FRP materials are provided withpolymeric resins such as polyesters, vinylesters, or epoxies, althoughother polymeric materials can also be used. Additionally, the fibers andthe FRP composites are heterogeneous and anisotropic which can makecharacterization and prediction of properties somewhat difficult.

The above described FRP composite structures can be produced inaccordance with a number of optional embodiments of the presentinvention. In one embodiment, existing structural columns can bereinforced. This is accomplished by placing a FRP outer shell around theexisting column such that there is an open space between the existingcolumn and the outer shell. Typically, this can be accomplished byplacing two pieces of a shell around the column. Once the outer shell isin place, at least one additional layer of FRP material is wrappedaround the outer shell to secure the two pieces together. Optionally,the outer shell can be formed and cured around the column while leavingan open space. The open space between the existing column and the outershell can then be filled with expansive or non-shrink cement.

In one embodiment of the present invention, the existing column has across sectional shape which is different than a cross-sectional shape ofthe outer shell. For example, the existing column may be rectangular inshape while the outer shell is circular or elliptical in shape.

Thus, there has thus been outlined, rather broadly, the more importantfeatures of the invention so that the detailed description thereof thatfollows may be better understood, and so that the present contributionto the art may be better appreciated. Other features of the presentinvention will become clearer from the following detailed description ofthe invention, taken with the accompanying drawings and claims, or maybe learned by the practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional features and advantages of the invention will be apparentfrom the detailed description which follows, taken in conjunction withthe accompanying drawings, which together illustrate, by way of example,features of the invention; and, wherein:

FIG. 1 is a perspective view of an exemplary FRP composite structureaccording to one embodiment of the present invention.

FIG. 2 is a cross sectional view of a FRP composite structure accordingto one embodiment of the present invention.

FIG. 3A is a cross sectional view of a FRP composite structure inaccordance with one embodiment of the present invention wherein theinner cement structure has a circular cross-sectional shape and the corehas a circular cross-sectional shape.

FIG. 3B is a cross sectional view of a FRP composite structure inaccordance with one embodiment of the present invention wherein theinner cement structure has a square cross-sectional shape and the corehas a circular cross sectional shape.

FIG. 3C is a cross sectional view of a FRP composite structure inaccordance with one embodiment of the present invention wherein theinner cement structure has a rectangular cross sectional shape and thecore has an elliptical cross sectional shape.

FIG. 4 is a perspective view of a FRP outer shell around an existingcolumn, with outer shell being configured to leave an open space betweenthe existing column and outer shell so that the open space may be filledwith expansive or non-shrink cement in accordance with one embodiment ofthe present invention.

FIG. 5 is a perspective view of a FRP composite outer shell which hasbeen divided into two pieces and place around an existing column inaccordance with one embodiment of the present invention.

FIG. 6 is a perspective view of two pieces of a FRP composite outershell that have been spliced with a vertical FRP composite strip alongthe seams between the two pieces in accordance with one embodiment ofthe present invention.

FIG. 7A is a perspective view of a mold used for forming a FRP compositeouter shell in accordance with one embodiment of the present invention.

FIG. 7B is a perspective view of a mold that has been partially wrappedwith at least one layer of fiber reinforced composite material to forman outer shell in accordance with one embodiment of the presentinvention.

FIG. 7C is a perspective view of an outer shell which has been dividedinto two pieces so that it can be used in applications requiringretrofitting existing columns in accordance with one embodiment of thepresent invention.

FIG. 8 is a graph of concrete strength vs. aging time in accordance withone embodiment of the present invention.

FIG. 9 is a graph of expansion hoop strain for expansive cement inaccordance with one embodiment of the present invention.

FIG. 10A is a graph of the expansion history of 12″ circular columns inaccordance with one embodiment of the present invention.

FIG. 10B is a graph of expansion history of 16″ circular columns inaccordance with one embodiment of the present invention.

FIG. 10C is a graph of expansion history of elliptical (1:2) columns inaccordance with one embodiment of the present invention.

FIG. 10D is a graph of expansion history of elliptical (1:3) columns inaccordance with one embodiment of the present invention.

FIG. 11A is a perspective view for a wrapping method of a circularcolumn in accordance with one embodiment of the present invention.

FIG. 11B is a cross sectional view of a circular column without a fiberreinforced polymer wrap.

FIG. 11C is a cross sectional view of a circular column with a fiberreinforced polymer wrap.

FIG. 11D is a cross sectional view of a circular column with a fiberreinforced polymer wrap.

FIG. 11E is a perspective view for a wrapping method of a circularcolumn in accordance with one embodiment of the present invention.

FIG. 11F is a cross sectional view of a circular column of expansiveconcrete with a fiber reinforced polymer wrap.

FIG. 11G is a cross sectional view of a circular column of expansiveconcrete with a fiber reinforced polymer wrap.

FIG. 11H is a perspective view for a wrapping method of a circularcolumn in accordance with one embodiment of the present invention.

FIG. 11I is a cross sectional view of a circular column with a fiberreinforced polymer wrap.

FIG. 11J is a cross sectional view of a circular column with a fiberreinforced polymer wrap.

FIG. 12A is a perspective view for a wrapping method of a square columnin accordance with one embodiment of the present invention.

FIG. 12B is a cross sectional view of a square column without a fiberreinforced polymer wrap.

FIG. 12C is a cross sectional view of a square column with a fiberreinforced polymer wrap.

FIG. 12D is a cross sectional view of a square column encircled byregular concrete and a fiber reinforced polymer wrap.

FIG. 12E is a cross sectional view of a square column with a fiberreinforced polymer wrap.

FIG. 12F is a cross sectional view of square column encircled by regularconcrete and a fiber reinforced polymer wrap.

FIG. 12G is a perspective view for a wrapping method of a in accordancewith one embodiment of the present invention.

FIG. 12H is a cross sectional view of a square column encircled byexpansive concrete and a fiber reinforced polymer wrap in accordancewith one embodiment of the present invention.

FIG. 12I is a cross sectional view of a square column encircled byexpansive concrete and a fiber reinforced polymer wrap in accordancewith one embodiment of the present invention.

FIG. 12J is a perspective view for a wrapping method of a square columnin accordance with one embodiment of the present invention.

FIG. 12K is a cross sectional view of a square column with a fiberreinforced polymer wrap.

FIG. 12L is a cross sectional view of a square column with a fiberreinforced polymer wrap.

FIG. 13A is a perspective view for a wrapping method of a rectangularcolumn in accordance with one embodiment of the present invention.

FIG. 13B is a cross sectional view of a rectangular column without afiber reinforced polymer wrap.

FIG. 13C is a cross sectional view of a rectangular column with a fiberreinforced polymer wrap.

FIG. 13D is a cross sectional view of a rectangular column encircled byregular concrete and a fiber reinforced polymer wrap.

FIG. 13E is a cross sectional view of a rectangular column with a fiberreinforced polymer wrap.

FIG. 13F is a cross sectional view of a rectangular column encircled byregular concrete and a fiber reinforced polymer wrap.

FIG. 13G is a perspective view for a wrapping method of a rectangularcolumn in accordance with one embodiment of the present invention.

FIG. 13H is a cross sectional view of a rectangular column encircled byregular concrete and a fiber reinforced polymer wrap.

FIG. 13I is a cross sectional view of a rectangular column encircled byregular concrete and a fiber reinforced polymer wrap.

FIG. 14A is a perspective view for a wrapping method of a rectangularcolumn in accordance with one embodiment of the present invention.

FIG. 14B is a cross sectional view of a rectangular column without afiber reinforced polymer wrap.

FIG. 14C is a cross sectional view of a rectangular column with a fiberreinforced polymer wrap.

FIG. 14D is a cross sectional view of a rectangular column encircledwith regular concrete and a fiber reinforced polymer wrap.

FIG. 14E is a cross sectional view of a rectangular column with a fiberreinforced polymer wrap.

FIG. 14F is a cross sectional view of a rectangular column encircled byregular concrete and a fiber reinforced polymer wrap.

FIG. 14G is a perspective view for a wrapping method of a rectangularcolumn in accordance with one embodiment of the present invention.

FIG. 14H is a cross sectional view of a rectangular column encircled byexpansive concrete and a fiber reinforced polymer wrap in accordancewith one embodiment of the present invention.

FIG. 14I is a cross sectional view of a rectangular column encircled byexpansive concrete and a fiber reinforced polymer wrap in accordancewith one embodiment of the present invention.

FIG. 15 is a side perspective view illustrating the placement ofspecimens in a compression machine in accordance with one embodiment ofthe present invention.

FIG. 16A is a cross sectional view illustrating placement of LVDTdevices in accordance with one embodiment of the present invention.

FIG. 16B is a cross sectional view illustrating placement of LVDTdevices in accordance with one embodiment of the present invention.

FIG. 16C is a cross sectional view illustrating placement of LVDTdevices in accordance with one embodiment of the present invention.

FIG. 17 is a perspective view of a column compression machine used intesting the specimens in accordance with one embodiment of the presentinvention.

FIG. 18 is a graph of load versus displacement behavior of circularspecimens.

FIG. 19 is a graph of stress versus strain relation for specimens withregular concrete.

FIG. 20 is a graph of stress versus strain relation for specimens withexpansive cement concrete.

FIG. 21 is a graph of stress versus strain for several specimens inaccordance with embodiments of the present invention.

FIG. 22 is a graph of stress versus strain for several specimens inaccordance with embodiments of the present invention.

FIG. 23 is a graph of finite element results for CFRP-confined squareand rectangular columns.

FIG. 24 is a graph of finite element results for CFRP-confinedelliptical columns.

The above figures are provided for illustration purposes and variationsin dimensions, shapes, materials and the like can be made withoutdeparting from the claimed scope of the invention.

Reference will now be made to the exemplary embodiments illustrated, andspecific language will be used herein to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended.

DETAILED DESCRIPTION

Before the present invention is disclosed and described, it is to beunderstood that this invention is not limited to the particularstructures, process steps, or materials disclosed herein, but isextended to equivalents thereof as would be recognized by thoseordinarily skilled in the relevant arts. It should also be understoodthat terminology employed herein is used for the purpose of describingparticular embodiments only and is not intended to be limiting.

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a,” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a layer” includes one or more of such layers, reference to“a column” includes reference to one or more of such structures, andreference to “a lay-up process” includes reference to one or more ofsuch processes.

Definitions

In describing and claiming the present invention, the followingterminology will be used in accordance with the definitions set forthbelow.

As used herein, “cement” as any material which can be used to bind. Forexample, concrete can include crushed stone, sand, and a cement.Portland cement is a fired mixture of limestone and clay which, whenhydrated, forms interlocking crystals which bind to the sand, stone, andone another. Cements can generally be classified as shrink, non-shrink,or expansive cements. The most commonly used cement for generalconstruction is shrink cement.

As used herein, “post-tension” refers to tension created or induced in amaterial subsequent to formation. For example, post-tensioning of FRPshells occurs after curing of the FRP shell to create a post-tensionedshell having circumferential, or hoop stress.

As used herein with respect to an identified property or circumstance,“substantially” refers to a degree of deviation that is sufficientlysmall so as to not measurably detract from the identified property orcircumstance. The exact degree of deviation allowable may in some casesdepend on the specific context.

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary. For example, glass fiber and carbon fiber are listed in acommon group. However, those skilled in the art will recognize thatglass fiber may be more or less suitable than carbon fiber for aspecific application depending on cost restrictions, strengthrequirements, and other factors.

Concentrations, amounts, and other numerical data may be expressed orpresented herein in a range format. It is to be understood that such arange format is used merely for convenience and brevity and thus shouldbe interpreted flexibly to include not only the numerical valuesexplicitly recited as the limits of the range, but also to include allthe individual numerical values or sub-ranges encompassed within thatrange as if each numerical value and sub-range is explicitly recited.

As an illustration, a numerical range of “about 1 inch to about 5inches” should be interpreted to include not only the explicitly recitedvalues of about 1 inch to about 5 inches, but also include individualvalues and sub-ranges within the indicated range. Thus, included in thisnumerical range are individual values such as 2, 3, and 4 and sub-rangessuch as from 1-3, from 2-4, and from 3-5, etc. This same principleapplies to ranges reciting only one numerical value. Furthermore, suchan interpretation should apply regardless of the breadth of the range orthe characteristics being described.

Invention

Due to the increasing need for repair of existing support structures,research has been carried out to investigate the behavior ofFRP-confined concrete columns. The compressive stress-strain behavior ofFRP confined concrete cylinders is generally nonlinear and the initialportion of the stress-strain response typically follows that of theunconfined concrete. Moreover, after reaching the peak unconfinedconcrete stress level, the response of the FRP-confined concretesoftens. This softening can occur with either a localized descendingbranch that may stabilize as the dilation of the concrete coreprogresses, or the concrete may exhibit a somewhat linear behavior untilthe FRP composite jacket fails.

In accordance with the present invention, FRP materials can be used toeither reinforce existing structures regardless of shape, or form newstructures with improved mechanical, structural and aestheticproperties.

Referring now to FIG. 1, a fiber reinforced composite structure 10 isshown comprising a core 14 including an inner cement structure 18 atleast partially surrounded by an outer cement structure 22. The innercement structure can have steel reinforcements therein; however, thepresent invention reduces the need for steel reinforcements. The outercement structure 22 may include a non-shrink cement or an expansivecement. Alternatively, the outer cement structure 22 may consistexclusively of expansive cement. Encircling the core 14 is an FRPmaterial 26, which makes up an FRP outer shell 30. The FRP material 26preferably comprises a fiber and polymeric matrix. Traditionally, thefiber is selected from the group consisting of glass fiber, carbonfiber, aramid fiber, and combinations thereof, although other FRPmaterials can also be used.

The composite structure 10 is shown in FIG. 2 with a core 14 having across sectional shape that is different from the cross sectional shapeof inner cement structure 18. Alternatively, the core 14 may have across sectional shape similar to the cross sectional shape of the innercement structure 18, as shown in FIG. 3A. The composite structure 10 isshown in FIGS. 3B and 3C having a core 14 with a cross sectional shapethat is different from the cross sectional shape of the inner cementstructure 18. Specifically, FIG. 3B shows the composite structure 10having a core 14 with a circular cross sectional shape and an innercement structure 18 with a square cross-sectional shape, and FIG. 3Cshows the composite structure 10 wherein the inner cement structure 18has a rectangular cross sectional shape and the core 14 has anelliptical cross sectional shape.

Now referring to FIG. 4, a FRP outer shell 30 is shown around anexisting column 34. The existing column can have steel reinforcementstherein; however, the present invention reduces the need for steelreinforcements. The outer shell 30 is configured such that there is anopen space 38 between the existing column 34 and the outer shell 30. Theopen space 38 can then be filled with cement 42. The cement 42 may beeither expansive cement or non-shrink cement or a combination thereof.In a preferred embodiment of the present invention, as shown in FIG. 1,pegs 15 can be placed between the inner cement structure and the FRPouter shell to secure the positioning of the outer column and existingcolumn prior to filling the open space with cement.

FIG. 5 illustrates one embodiment of the present invention wherein theouter shell 30 comprises at least two pieces which can be placed aroundthe existing column 34 to form the outer shell 30. This can be achievedby separating the FRP outer shell 30 into two pieces and placing the twopieces around the existing column to form the outer shell. To reinforcean existing column 34 it is typically necessary to separate the outershell 30 longitudinally into a first piece 46 and a second piece 48. Thefirst piece 46 and second piece 48 can then be placed around theexisting column 34 to reform the outer shell 30. In most cases, theouter shell 30 is designed and shaped to leave an open space 38 betweenthe existing column 34 and the outer shell 30. Thus, the outer shell 30provides a convenient avenue for shape modification of existingstructures. Shape modification is particularly relevant with respect toretrofitting existing structures. An existing column 34 with a square orrectangular cross-section may be modified such that the resultingcomposite structure 10 has a circular or elliptical cross-section.Typically, FRP composite jackets subjected to membrane loading inaccordance with the present invention are stronger than rectangularcolumn sections having long flat sides. This is largely because of thedominant bending action of the flat sides. Therefore, shape modificationof an existing column 34 using the present invention can be readilyaccomplished to provide improved structural and mechanical properties tothe composite structure 10.

Once the first piece 46 and second piece 48 have been placed around theexisting structure 34 they can be spliced, as shown in FIG. 6, with avertical FRP composite strip 56 along each seam 58 between the firstpiece 46 and second piece 48 so as to form a unitary outer shell 52. Ina preferred embodiment of the present invention, after the first andsecond piece of the outer shell have been spliced with a vertical FRPcomposite strip, additional FRP material may be wrapped around the outershell. Typically the wrapping can be done with a single continuoussheet; however, multiple sheets can be wrapped in a wet lay-up processfollowed by curing of the polymer resin. Most often, the number oflayers can range from 1 to about 14 additional layers.

In yet another preferred embodiment of the present invention, prior toplacing the first piece 46 and second piece 48 around the existingcolumn 34, the existing column may be reshaped such that the edges ofthe existing column having an angle of about 90 degrees are rounded. Oneimportant consideration in forming FRP-confined rectangular columns inaccordance with the present invention is the issue of effectiveness ofFRP confinement, which may significantly decrease due to the presence of90° corners or abrupt change of direction around the perimeter.Small-scale tests illustrate the effect of rounding the column comers onconfinement efficiency. The rounding of corners on concrete columns hasbeen shown to have an effect in ultimate strength as high as an 80%increase with respect to square columns without rounding the corners. Inaddition to higher strengths, higher ultimate compressive strains can beachieved for columns with rounded comers, which is more important forseismic applications than strength. Typical ultimate strain increasesrange from 200% to 300%. While the effectiveness of confinementincreases with the corner radius, the rounding of corners cannot alwaysbe made in practice as large as ideally desired because of the presenceof the hoop steel reinforcement, typically about 1.5-2.5 in. from theexterior concrete surface.

In one embodiment of the invention, FRP materials can be placed alongthe longitudinal axis of the existing column in direct contact with theexisting column, either during or after formation of the FRP shell, forincreased flexural resistance of the column, if required.

Many applications will call for a FRP outer shell that ispre-manufactured. However, many applications will require manufacture ofthe outer shell. In these applications, a mold can be prepared in orderto form the FRP outer shell. A mold can be prepared to correspond with adesired final shape of the column. A mold is not necessarily the sameshape as the existing column. Frequently, an existing rectangular orsquare column can be modified to produce a circular or elliptical columnof slightly larger width.

In one embodiment of the present invention as illustrated in FIGS. 7A,7B and 7C, a mold 60 is prepared. The mold 60 is then wrapped with atleast one layer of FRP material 64 to form an outer shell 72. The moldcan then be removed leaving the outer shell as an independent structure.Typically, the wrapping can be done with a single continuous sheet;however, multiple sheets can be wrapped in a wet lay-up process followedby curing of the polymer resin. The sheets may be cut to a desired sizeor as lengths of fiber strands, as shown in FIG. 7B, which can bewrapped and/or laid as desired to form a particular shape. Most often,the number of layers can range from 1 to about 14 additional layers.

In a preferred embodiment of the present invention, the FRP material 64comprises a fiber and a polymeric matrix. Typical fibers can include,but are not limited to, glass fiber, carbon fiber, aramid fiber, andcombinations thereof. Any suitable FRP material can be used whichincludes a fiber material and a polymeric matrix. Non-limiting examplesof commercial products can include SikaWrap, Aquawrap, and the like.

Wrapping of the mold 60 may include a wet layup of resin coated fibersfollowed by a curing of the resin. Once the resin has cured, the outershell 72 can be divided longitudinally into at least a first piece 76and a second piece 78 so that it can be used in applications thatrequire retrofitting existing columns.

For purposes of designing FRP structures for existing columns, thosepersons skilled in the art will recognize that it is possible to usefinite element analysis to prepare the design of the FRP structure priorto retrofitting the existing column.

EXAMPLES

The following examples illustrate exemplary embodiments of theinvention. However, it is to be understood that the following is onlyexemplary or illustrative of the application of the principles of thepresent invention. Numerous modifications and alternative compositions,methods, and systems may be devised by those skilled in the art withoutdeparting from the spirit and scope of the present invention. Theappended claims are intended to cover such modifications andarrangements. Thus, while the present invention has been described abovewith particularity, the following examples provide further detail inconnection with what is presently deemed to be practical embodiments ofthe invention.

In the following examples, the radii of curvature for the 90 degreecorners of the square and rectangular columns were designed to be ¾ in.This would allow modification of existing columns, taking into accounttypical existing steel reinforcement at 90 degree corners. Expansivecement was used for some examples, whereas non-shrink cement was usedfor other examples to fill the space between the outer shell andexisting column.

Further, regular shrink concrete was used to prepare a number of testsamples which were then compared to samples using expansive andnon-shrink concrete. All columns were cast in one batch to eliminatevariations between them; 6″×12″ and 4″×8″ standard cylinders were madealong with these specimens. The compressive strength was obtained fromthe tests of cylinders at 28 days after casting the concrete. Theconcrete strength versus time relationship is shown in FIG. 8. FIG. 8illustrates that the concrete strength increased during the first 6months and after six months approaches a constant value of 2600 psi.

Expansive Cement

Unstressed FRP composite jackets do not participate in the confinementof concrete until the concrete starts expanding. Typically, thisinvolves at least partial failure of the concrete and/or softening ofthe concrete. In accordance with the present invention, expansiveconcrete can post-tension the FRP composites jacket in the hoopdirection prior to application of vertical or axial loading to thecolumn.

The expansive cement used in the following examples includes Type-K andKOMPONENT cements, manufactured by CTS Company, Cypress, Calif. The twoprincipal constituents of KOMPONENT are calcium sulfoaluminate andgypsum (calcium sulfate). The formation of ettringite crystals, whichresults from the hydration of the two ingredients, causes an expansionof the cement. When expansion is restrained by FRP composite jackets inaccordance with the present invention, the expansive cement inducestensile stress in the FRP composite jackets along the circumference ofthe FRP jackets.

To determine the optimal mixing ratio of Type-K cement and KOMPONENT, apreliminary test was conducted. In this test, four types of expansivecements were investigated:

(1) MIX 1: 100% Expansive KOMPONENT

(2) MIX 2: 75% Expansive KOMPONENT+25% Portland Cement

(3) MIX 3: 50% Expansive KOMPONENT+50% Portland Cement

(4) MIX 4: 15% Expansive KOMPONENT+85% Portland Cement

Each of the four mixtures was prepared and cast in prefabricated CFRPcylinder shells (6″×12″). Strain gauges were placed at the middle heightof the CFRP cylinders to monitor the expansion history over time. Theresulting strain curves are shown in FIG. 9, which shows that both mix(2) and mix (3) gave the largest hoop strain for the FRP jacket comparedto the other two mixes. Data from accompanying cylinder tests show thatthe compressive strength of mix (3) was 920 psi compared to 850 psi formix (2). Mix (3) was selected since it contained less expansive cement.Thus, it appears that for this particular expansive cement, volumepercents of expansive cement from about 30 to about 85 can be suitable.The final mix design (based on mix (3)) for the expansive concrete isshown in Table 1. TABLE 1 Mix Design for the Expansive ConcreteCOMPONENT DETAILS Gals/% lbs C.F. Cement Type K 10% 351 1.79 ExpansiveCement Komponent  5% 166 0.84 Water U.S. Gallons 45 gall 375 6.00 Air %Entrained 7% +/− 1% 1.89 Air Target Rock ASTM C-33 520 3.22 (SSD) ⅜s PeaGravel Sand ASTM C-33 2143 13.26 (SSD) Total 3555 25.3.00

Several specimens with 12 in. diameter circular, 16 in. diametercircular and elliptical (aspect ratio 1:2 and 1:3) with CFRP and GFRPjackets, were cast and then cured at an indoor temperature around 70° F.The details of construction are described below in more detail. A dataacquisition system was used to monitor the hoop expansion of the FRPcomposite jacket for each specimen. FIGS. 10A-10D show the hoopexpansion strain of the circular and elliptical columns for about 70days.

Non-Shrinkage Concrete

As an alternative to expansive cement, a shrinkage compensated cementwas also used to compare the differences between the expansive cementFRP jackets and the non-shrinkage FRP jackets. In this case, thenon-shrinkage concrete was used to modify the rectangular or squaresections to elliptical or circular sections as detailed below. Once theaging of the concrete was stable, the FRP jackets were wrapped using awet lay-up process. A structural grout, Sika Grout 212 was selected asthe grout to make non-shrink concrete.

Fiber Reinforced Polymer

Two FRP composite materials were used to confine the concrete columns.One was SikaWrap Hex 103C (available from Sika Canada Inc.), which is ahigh strength, unidirectional carbon fiber fabric. The second FRPmaterial was Aquawrap G-06 by Air Logistics, which is a unidirectionalpre-impregnated glass fiber fabric. The primary material propertiesdetermined in this study are shown in Table 2. TABLE 2 MaterialProperties of CFRP and GFRP Composites* FRP Tensile Tensile Tensile PlyThick- Composite Strength (ksi) Modulus (Msi) Strain (%) ness (in.) CFRP177 12.6 1.4 0.038 GFRP 33 2.45 1.4 0.064*Determined at University of Utah, following ASTM D3039 after curing atroom temperature.Construction of Specimens and Sample Preparation

A total of 30 test structures were prepared such that each had nearlythe same cross-sectional area prior to shape modification and the sameheight of 3 feet. Thus, comparisons were made for different crosssections and aspect ratios. Molds for specimens were made out of plywoodand sonatubes.

In addition, both CFRP and GFRP composites-confined specimens weretested and compared with baseline specimens without FRP composites. Abreakdown of the test matrix is presented in Tables 3-6. FIGS. 11Athrough 11J, 12A through 12L, 13A-13I, and 14A-14I show details of thewrapping methods for each specimen. Regardless of the number of FRPlayers, the entire bonded jackets were made of one continuous sheet ofFRP fabric that was cut to the proper length and width. An additional 3″of overlap splice was provided. To expansive cement concrete specimens,the following steps were followed:

(a) Preparation of circular and elliptical forms.

(b) Build 1^(st) layer of FRP shell and cut the shell into two halves.

(c) Make stay-in-place FRP forms by lap splicing with one FRP layer andapplying the other remaining layers.

(d) Pour expansive cement concrete to fill the open space between theFRP shell and the standard concrete column. TABLE 3 Matrix of TestSpecimens: Circular Columns COLUMN INI- INI- DESIG- TIAL TIAL NO. NATIONTYPE LENGTH SIZE FRP TYPE Note 1 C-0-0 Circular 36″ 12″ None 2 C-C1-0Circular 36″ 12″ Carbon 1 Layer 3 C-G3-0 Circular 36″ 12″ Glass 3 Layers4 C-CT-E Circular 36″ 12″ Carbon expansive Fiber concrete Tube (1 layer)5 C-GT-E Circular 36″ 12″ Glass expansive Fiber concrete Tube (3 layers)6 C-CS-0 Circular 36″ 12″ Carbon 2 Layers Fiber of CFRP Strip 7 C-GS-0Circular 36″ 12″ Glass 6 Layers Fiber of GFRP Strip

TABLE 4 Matrix of Test Specimens: Square Columns COLUMN INITIAL INITIALNO. DESIGNATION TYPE LENGTH SIZE FRP TYPE Note 8 S-0-0 Square 36″ 11″ ×11″ None 9 S-C2-0 Square 36″ 11″ × 11″ carbon 2 layers ¾″ radii at 90°corners 10 S-C2-F Square 36″ 11″ × 11″ carbon 2 layers 11 S-G6-0 Square36″ 11″ × 11″ Glass 6 layers ¾″ radii at 90° corners 12 S-G6-F Square36″ 11″ × 11″ Glass 6 layers 13 S-CT-E Square 36″ 11″ × 11″ Carbon fibertube (2 layers) 14 S-GT-E Square 36″ 11″ × 11″ Glass fiber tube (6layers) 15 S-CS-0 Square 36″ 11″ × 11″ Carbon fiber ¾″ radii strip at90° corners 16 S-GS-0 Square 36″ 11″ × 11″ Glass fiber ¾″ radii strip at90° corners

TABLE 5 Matrix of Test Specimens: Rectangular Columns (1) COLUMN INITIALINITIAL NO. DESIGNATION TYPE Length SIZE FRP TYPE Note 17 R2-0-0Rectangular 36″ 8″ × 15″ None 18 R2-C2-0 Rectangular 36″ 8″ × 15″ carbon2 layers ¾″ radii at 90° corners 19 R2-C2-F Rectangular 36″ 8″ × 15″carbon 2 layers 20 R2-G6-0 Rectangular 36″ 8″ × 15″ Glass 6 layers ¾″radii at 90° corners 21 R2-G6-F Rectangular 36″ 8″ × 15″ Glass 6 layers22 R2-CT-E Rectangular 36″ 8″ × 15″ Carbon fiber tube (2 layers) 23R2-GT-E Rectangular 36″ 8″ × 15″ Glass fiber tube (6 layers)Note:For the elliptical columns, the longer axis is 21.2 in. and the shorteraxis is 11.4 in.

TABLE 6 Matrix of Test Specimens: Rectangular Columns (2) COLUMN INITIALINITIAL NO. DESIGNATION TYPE Length SIZE FRP TYPE Note 24 R3-0-0Rectangular 36″ 6″ × 18″ None 25 R3-C2-0 Rectangular 36″ 6″ × 18″ carbon2 layers ¾″ radii at 90° corners 26 R3-C2-F Rectangular 36″ 6″ × 18″carbon 2 layers 27 R3-G6-0 Rectangular 36″ 6″ × 18″ Glass 6 layers ¾″radii at 90° corners 28 R3-G6-F Rectangular 36″ 6″ × 18″ Glass 6 layers29 R3-CT-E Rectangular 36″ 6″ × 18″ Carbon fiber tube (2 layers) 30R3-GT-E Rectangular 36″ 6″ × 18″ Glass fiber tube (6 layers)Note:For the elliptical columns, the longer axis is 25.4 in. and the shorteraxis is 8.4 in.

Testing of Specimens

The strain gauges employed in this testing program were manufactured bymeasurements Group, Inc. with model designation as EA-06-125BZ-350. Theresistance of these gauges at normal temperature (75° F.) is 350±0.15%ohms. In order to measure the transverse strain on the FRP jacket duringloading, strain gauges were placed on fibers in the hoop direction, atabout the mid-height of the specimens. Special care was taken during theinstallation to avoid damage of the strain gauges. Considering thegeometry of the cross sections, the layout of strain gauges was variedfor each shape.

Linear variable differential transducers (LVDTs) are used to measureaverage strains when the use of strain gauges is impossible. In thesetests, LVDTs are employed to measure the vertical and lateral strains.The data from LVDTs can be used to calculate the average axial andtransverse strains over the column height and width. LVDTs are installedusing aluminum angles with threaded rods at their ends. The angles mustbe solidly clamped to the specimen for accurate readings. FIG. 15 showsthe LVDT configuration. Two types of LVDTs used for experiments areMVL7C and MVL7, manufactured by Sensotec Company. They can measure adisplacement in the range of ±0.500 in. and ±2.000 in., respectively,with high accuracy. For the square and rectangular columns, additionalLVDTs are installed on the two sides of the cross section to measure thetransverse strain in both directions as shown in FIGS. 16A-16C.

The specimens were tested using a structural load frame having anactuator manufactured by Geneva Hydraulics, Inc. This actuator canimpose a compression load up to 2000 kips and is capable of a 24 in.stroke. FIG. 17 illustrates the setup of the column compression tests.All of the specimens were loaded monotonically under a displacementcontrol mode with a constant loading rate of 0.05 in. per minute.

A data acquisition system was used to record the values of the straingauges and LVDTs. The data acquisition system used in this testingprogram consisted of scanners, WIN5100 (manufactured by MeasurementsGroup) with interface cards and the STRAINSMART software. The scannerscan read electrical signals from the sensors and send this informationto a computer via the interface cards. The software then converts thesesignals into the desired digital output. Prior to the start of testing,a configuration file had to be written in the software to assign themeasured quantities to input and output channels. In addition, thecalibration values of strain gauges and LVDTs were input into thisconfiguration file. After the setup of the configuration file andimmediately before testing, all of the initial values were set to zeroto prepare for recording.

Experimental Results

In early tests, a group of 7 circular columns were tested. All of theFRP strengthened specimens showed significant increases in axial stressand axial strain capacity. As seen in Table 7, the increase in ultimatestrength (f′_(cc)/f′_(co)) ranges from 138 to 238 percent for regularconcrete specimens and 545 to 676 percent for expansive cement concretespecimens. In addition, compared with the baseline column without anyreinforcement, FRP composite jackets improve the confinement of thecolumns which results in a significant increase in axial strain aspresented in Table 7. The increases in ultimate strain (ε′_(cc)/ε′_(co))ranges from 848 to 1421 percent for the FRP reinforced specimens. TABLE7 Results of Circular Column Tests Wrapping f′_(cc)/ Specimen No. FRPType Layers Method f′_(co) ε′_(cc)/ε′_(co) C-0-0* NO FRP 1.00 1.00C-C1-0* CFRP 1 Continuous 2.20 8.48 (wet-layup) C-G3-0* GFRP 3Continuous 2.38 12.06 (wet-layup) C-CS-0* CFRP 2 5 in. Strips 1.38 9.36(wet-layup) (5 in. spacing) C-GS-0* GFRP 6 5 in. Strips 1.82 14.21(wet-layup) (5 in. spacing) C-CT-E** CFRP 1 Continuous 5.45 11.74(prestressed) C-GT-E** GFRP 3 Continuous 6.76 13.71 (prestressed)*Regular concrete,**Expansive cement concrete

The specimens in this testing group exhibited several failure modes. Thegoverning failure mode was determined by the mechanical properties ofthe FRP composite material and the reinforcement scheme. It was observedfrom the tests that the most typical failure mechanism was crushing ofconcrete followed by the tensile failure of the FRP at or near themid-height of the specimens. Because the fabric was unidirectional andoriented at 0 degrees, a band or ring was typically formed as a resultof the shearing off, and separation of, the fabric in the hoopdirection.

The load-versus-displacement graphs for each specimen are presented inFIG. 18. As well as the increased axial strain, this increaseddeflection is also believed to be a result of the greater energyabsorption capacity of the specimens provided by the FRP composites.These results indicate that application of high performance FRPcomposites to a concrete member does not promote brittle failure.Therefore, the ductility of FRP-confined specimens can be described byusing the principle of energy absorption by comparing the area under theload-displacement curves.

The stress-strain curves for regular concrete specimens and specimenswith expansive cement concrete are shown in FIG. 19 and 20,respectively. As seen from FIG. 19, the loading behavior of theFRP-confined specimens with regular concrete can be divided into threephases. The first phase is from the origin to point A. Point Acorresponds to an axial stress f=0.8f_(co)′ (where f_(co)′ is thestrength of the baseline specimen C-0-0). In this phase where thebehavior for all specimens (in FIG. 19) is almost the same, lateralexpansion was very small and the FRP stresses were very low (about16%-33% of their ultimate strength as observed from the tests). When theload exceeded point A, the FRP stress and strain started to increasequickly. Cracking noises were heard once the loading approached f_(co)′which marked the FRP being put into tension. In the second phase, from Ato B, the FRP composite participated in confinement which resulted inthe small axial strain (displacement) as seen from stress-versus-strainand load-versus-displacement curves. After point B, which marked theturning point on the stress-strain curves, the loading went into thethird phase. In this phase, the FRP strain increased very quickly andthe specimen stiffness decreased as observed form the tests. Thespecimens deformed largely in the axial direction and the fabric wasdeformed on the surface of the FRP composite. Finally, the concrete inthe specimens crushed followed by the fracture of FRP.

These three phases represent the typical behavior of the axially loadedFRP-confined concrete specimens. In the beginning phase, the concretehas a small expansion and the FRP jacket does not participate. However,in phase AB, the concrete expands and FRP composite jacket is put intotension. Therefore, the FRP material provides partial confinementagainst expansion. In the last phase after B, the concrete goes into aflowing state until FRP fracture and the failure is very brittle. Thebehavior of the specimens with expansive cement concrete was somewhatdifferent. Specifically, the initial slope of the stress-strain curve islower than that of the regular concrete specimens, but the later slopeafter the turning point is similar to that of the regular concretespecimens. The comparisons show that the axial stress capacity wasequivalent to the FRP-confined regular concrete specimens and the axialstrain capacity was much larger.

Testing results for several square columns are shown in FIGS. 21 and 22.In each figure S-0-0 shows results for an 11 in. square column withoutFRP composites, S-G6-0 shows results for an 11 in. square column with 6layers of glass FRP composite directly wrapped thereon, and S-GT-E showsresults for an 11 in. square column with expansive concrete modifiedinto a 16 in. diameter circle with 6 layers of glass FRP composite inaccordance with the present invention. FIG. 21 illustrates that the GFRPexpansive composite column has a strength of about 3.3 times that of theunwrapped column and about 2.3 times that of the directly wrappedcolumn. Likewise, FIG. 22 illustrates that strain, i.e. ductility, forthe GFRP expansive composite column is about 8 times that of theunwrapped column.

Thus, the above discussion illustrates that FRP composites are veryeffective in increasing the load-carrying capacity and deformationability of existing columns. Significant increases in both ultimatestress and strain are observed from the tests. When compared to theregular concrete specimens, the specimens with expansive cement concreteshow more deformation ability and ductility at failure. In addition,they have a higher increase in the ultimate strength. With the samevolumetric FRP ratio, the confinement effectiveness for specimensconfined by FRP strips decreases if compared to those confined withcontinuous FRP. Therefore, the strengthening method with FRP stripsshould be used with caution for the normal retrofit of bridge columns.Otherwise, the maximum spacing of FRP strips should be limited. However,strengthening with FRP strips offers the advantage of easy inspection.The FRP composites can significantly improve the axial behavior of thecolumns. It is recommended that at least two layers of FRP composites beused for the retrofit of existing columns.

Finite Element Analysis

To validate the results obtained from the experimental research, anonlinear finite analysis was conducted by using the finite elementsoftware package: ANSYS6.0 (ANSYS 2000). Four types of models weredeveloped according to the geometric characteristics of the specimens.SOLID65, which is an eight-node brick element with 3 DOFs at each node,was used to model concrete. SHELL181 is a four-noded element that iswell-suited to model FRP composite materials. The material propertiesfor the unconfined concrete and FRP composites were obtained fromcompression cylinder and tensile coupon tests, respectively. Consideringthe symmetry of each column, only one-quarter of the column sectionalong its longitudinal direction was modeled; symmetrical boundaryconditions were applied at the symmetrical borders along the X and Yaxes. To model the pre-tensioning effect of the expansive cementconcrete on the FRP jackets, an equivalent thermal gradient was appliedon the FRP composite jacket to obtain the pre-stressed hoop strain priorto applying the axial loading. For each model, a nonlinear analysis wasconducted considering both material and geometric nonlinear behavior.The loading process was divided into many incremental steps, in which anincremental axial displacement was applied. Every increment was iterateduntil convergence was met with respect to the criteria of force anddisplacement. To optimize the calculation, each load step was dividedinto 20 sub-steps to expedite the process of convergence.

Analysis Results

The output of ANSYS results consists of the nodal displacement, elementstress, element strain and other information. For the axial stress ineach loading step, a mean value was calculated by taking the average ofthe longitudinal 6 elements along the central line of the model. Byentering this value into the element result tables, circumferential orhoop strain was obtained. Then, the curves of axial stress versus axialstrain and hoop strain were developed by joining a series of data foreach loading step.

The results of finite element analysis for some rectangular/squarespecimens are summarized in the stress-strain curves in FIGS. 23 and 24.It is noted from FIG. 23 that the confinement provided was notsufficient to significantly increase the axial stress for the square andrectangular sections. Both the square and rectangular modelsdemonstrated a typical softening behavior which is characterized by asudden drop from the peak stress (as seen from FIG. 23). Observationsfrom the results of the normal elliptical models (as shown in FIG. 24)also demonstrated the softening behavior. However, this softening is notas much as that of the rectangular columns. This result illustrates thatthe confinement effect is directly related to the shape of the sectionand that the confinement efficiency of circular columns is much betterthan the sections with 90 degree corner radius. Another important resultcan be observed from the comparison of bonded and pre-stressed FRPjackets of the present invention, as shown in FIG. 24, that theeffectiveness of prestressing on the FRP also has a significantcontribution to the confinement efficiency. This type of finite elementanalysis can also be used to design a retrofit to suit a specificproject by choosing the desired increase in strength and efficiency andvarying parameters such as shape and number of layers to balance resultswith cost effectiveness.

Of course, it is to be understood that the above-described arrangementsare only illustrative of the application of the principles of thepresent invention. Numerous modifications and alternative arrangementsmay be devised by those skilled in the art without departing from thespirit and scope of the present invention and the appended claims areintended to cover such modifications and arrangements. Thus, while thepresent invention has been described above with particularity and detailin connection with what is presently deemed to be the most practical andpreferred embodiments of the invention, it will be apparent to those ofordinary skill in the art that numerous modifications, including, butnot limited to, variations in size, materials, shape, form, function andmanner of operation, assembly and use may be made without departing fromthe principles and concepts set forth herein.

1. A fiber reinforced polymer composite structure, comprising: a) a coreincluding an inner cement structure at least partially surrounded by anouter cement structure, said outer cement structure including anon-shrink cement or an expansive cement; and b) a fiber reinforcedpolymer material at least partially surrounding said core.
 2. Thecomposite structure of claim 1, wherein the inner cement structure has across-sectional shape which is different than a cross sectional shape ofthe core.
 3. The composite structure of claim 2, wherein the innercement structure has a rectangular shape and the core has a circularshape.
 4. The composite structure of claim 1, wherein the outer cementstructure consists essentially of an expansive cement.
 5. The compositestructure of claim 1, wherein the core is post-tensioned and has a hoopstress along the fiber reinforced polymer material.
 6. The compositestructure of claim 1, wherein the fiber reinforced polymer materialcomprises a fiber and a polymeric matrix.
 7. The composite structure ofclaim 6, wherein the fiber is selected from the group consisting ofglass fiber, carbon fiber, aramid fiber, and combinations thereof. 8.The composite structure of claim 7, wherein the fiber is carbon fiber.9. A method of reinforcing structural columns, comprising the steps of:a) placing a fiber reinforced polymer outer shell around an existingcolumn, said outer shell being configured to leave an open space betweenthe existing column and the outer shell; b) filling the open space withexpansive cement or non-shrink cement.
 10. The method of claim 9,wherein the existing column has a cross-sectional shape which isdifferent than a cross-sectional shape of the outer shell.
 11. Themethod of claim 9, wherein the fiber reinforced polymer outer shellcomprises a fiber and a polymeric matrix.
 12. The method of claim 11,wherein the fiber is selected from the group consisting of glass fiber,carbon fiber, aramid fiber, and combinations thereof.
 13. The method ofclaim 9, wherein the outer shell comprises at least two pieces which areplaced around the existing column to form the outer shell.
 14. Themethod of claim 13, wherein at least one additional layer of fiberreinforced polymer material is wrapped around the outer shell afterplacing the fiber reinforced polymer outer shell around the existingcolumn.
 15. The method of claim 13, further comprising the step ofsplicing the at least two pieces with a vertical fiber reinforcedpolymer composite strip along each seam between the at least two pieces.16. The method of claim 9, further comprising the step of reshaping theexisting column prior to placing the fiber reinforced polymer outershell such that edges of the existing column having an angle of about 90degrees are rounded.
 17. The method of claim 9, further comprising thesteps of: a) preparing a mold b) wrapping the mold with at least onelayer of fiber reinforced polymer material to form the outer shell; c)dividing the outer shell longitudinally into at least two pieces; 18.The method of claim 9, further comprising the step of designing thefiber reinforced outer shell prior to the step of placing the fiberreinforce outer shell around the existing column, said step of designingincluding: a) defining a core finite element model in three dimensionscorresponding to the core of the composite structure; b) defining ajacket finite element model along an outer surface of the core finiteelement model; c) defining boundary conditions for each of the core andjacket finite element models; d) post-tensioning the jacket finiteelement model by applying an equivalent thermal gradient; and e)performing finite element analysis by incremental application of asimulated load and subsequent iteration calculate force and displacementof each node within each finite element model to form a stress-straincurve; and f) comparing the stress-strain curve to a desired performanceand redefining the jacket finite element model and the boundaryconditions when the stress-strain curve does not meet the desiredperformance.
 19. A method of preparing fiber reinforced polymer shellsfor reinforcing structural columns, comprising the steps of: a)preparing a mold; b) wrapping the mold with at least one layer of fiberreinforced polymer material to form an outer shell; and c) dividing theouter shell longitudinally into at least two pieces.
 20. The method ofclaim 19, wherein the step of wrapping the mold includes a wet layup ofresin coated fibers followed by curing of the resin.
 21. The method ofclaim 19, wherein the fiber reinforced polymer material comprises afiber and a polymeric matrix.
 22. The method of claim 21, wherein thefiber is selected from the group consisting of glass fiber, carbonfiber, aramid fiber, and combinations thereof.
 23. The method of claim21, wherein the fiber is in the form of a sheet or a strand.